Tanks and valves work together, so we'll look at them together.Even a scuba nondiver knows that a scuba tank is a cylindrical metal container used to safely store high-pressure air so you have something to breathe underwater. Almost as obvious is the purpose of the tank valve, which is to control air flow from the tank. Sounds simple, but what you may not realize is that there are different types of tanks and valves to handle these two simple jobs.

Tanks style and features Tanks come in a varietv of air capacities, depending upon their pressure rating and size. In the metric system, you express tank capacity in litres or kilograms of water capacity. The most common sizes are 8, 10, 12 and 15 litres. In the imperial system, you express capacity in the number of cubic feet of air you would have if you released it all at the surface. The three most common tank sizes are 50, 71.2 and 80 cubic feet, although other sizes are available.
 


 

The standard 12 litre/ 71.2 or 80-cubic-foot tank contains about the same air you have in a walk-in closet, compressed into a space about 600 mm/two feet long and 150 mm/half a foot in diameter. 

As this air is compressed into the tank, its pressure increases. The pressure in scuba tanks may be higher than 320 bar/4500 pounds per square inch (psi), but typical pressure ratings range from about 170 to 200 bar, or 2250 and 3000 psi.

Scuba tanks are either made of aluminum or steel. Both types are subject to regulations usually established by govern-ment agencies such as the U.S. Department of Transportation (DOT), Canadian Transport Commission (CTC) and similar agencies in other countries. Among these regulations, scuba tanks must pass periodic pressure tests (discussed below) mandated by these agencies.